Use Thread to support email delivery in background

[onlyphantom]

Prevent the server from waiting (thus slowing down) while the email delivery (eg. Reset Password request) is being sent.

Old:

from flask_mail import Mail
def send_email(subject, sender, recipients, message):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.html = message
    mail.send() # mail = Mail(app)

New:

def send_email_bg(app, message):
    with app.app_context():
        mail.send(message)

def send_email(subject, sender, recipients, message):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.html = message
    Thread(target=send_email_bg, args=(app, msg)).start()